Various materials are used as the active materials for the negative electrodes of non-aqueous electrolyte secondary batteries, such as lithium ion secondary batteries. Among them, graphite materials are mainly used, and natural graphite, artificial graphite, graphitized mesophase carbon particles, graphitized mesophase carbon fibers, etc. are used. Among them, active material particles with a graphite structure, such as natural graphite, artificial graphite, and graphitized mesophase carbon particles, are promising.
In the case of using active material particles with a graphite structure, the active material particles and a binder are mixed in the presence of a predetermined dispersion medium, to form a negative electrode mixture slurry. Examples of binders usually used include: latex containing rubber particles such as styrene butadiene rubber (SBR), polybutadiene, or nitrile-butadiene polymer (NBR); and cellulose ether compounds such as carboxymethyl cellulose (CMC), carboxy ethyl cellulose (CEC), sodium salts thereof, and potassium salts thereof (PTLs 1 to 3). The negative electrode mixture slurry is applied onto a negative electrode core member comprising a copper foil or the like, and dried to form a negative electrode mixture layer. Thereafter, the negative electrode mixture layer is rolled with reduction rolls. The negative electrode mixture layer integrated with the negative electrode core member is cut to a predetermined shape, to obtain a negative electrode.
A lithium-containing transition metal oxide such as LiCoO2, LiNiO2, or LiMn2O4 is used as a positive electrode active material. The positive electrode active material is mixed with a predetermined dispersion medium, a binder, and a conductive agent to form a positive electrode mixture slurry. The positive electrode mixture slurry is applied onto a positive electrode core member comprising an aluminum foil or the like, dried, rolled, and cut in the same manner as the negative electrode, to obtain a positive electrode.
The negative electrode and the positive electrode thus obtained are opposed with a separator interposed therebetween, wound spirally, and placed in a case of a predetermined shape together with a non-aqueous electrolyte, to obtain a lithium ion secondary battery. The non-aqueous electrolyte is prepared by dissolving a lithium salt such as LiPF6, LiClO4, or LiBF4 in a non-aqueous solvent such as ethylene carbonate (EC) or propylene carbonate (PC).
When such a battery is charged/discharged repeatedly, the active material particles with a graphite structure contained in the negative electrode expand and contract repeatedly, thereby weakening the bonding strength between the active material particles. As a result, the gaps between the active material particles may gradually increase, thereby causing the thickness of the negative electrode mixture layer to increase and causing the case to swell. To prevent swelling of the case due to an increase in the thickness of the negative electrode mixture layer, it is necessary to increase the bonding strength between the active material particles.